U.S. patent number 4,252,613 [Application Number 05/928,121] was granted by the patent office on 1981-02-24 for nuclear fuel assembly guide tube with integral intermittent projections.
This patent grant is currently assigned to The Babcock & Wilcox Company. Invention is credited to Felix S. Jabsen.
United States Patent |
4,252,613 |
Jabsen |
February 24, 1981 |
**Please see images for:
( Certificate of Correction ) ** |
Nuclear fuel assembly guide tube with integral intermittent
projections
Abstract
A nuclear fuel assembly includes guide tubes having integral
ridges oriented and spaced to increase coolant flow in the gap
between the guide tubes and adjacent fuel elements.
Inventors: |
Jabsen; Felix S. (Lynchburg,
VA) |
Assignee: |
The Babcock & Wilcox
Company (N/A)
|
Family
ID: |
25455763 |
Appl.
No.: |
05/928,121 |
Filed: |
July 26, 1978 |
Current U.S.
Class: |
376/439;
976/DIG.60 |
Current CPC
Class: |
F28F
13/06 (20130101); G21C 3/322 (20130101); Y02E
30/38 (20130101); Y02E 30/30 (20130101) |
Current International
Class: |
F28F
13/00 (20060101); G21C 3/32 (20060101); F28F
13/06 (20060101); G21C 3/322 (20060101); G21C
003/30 () |
Field of
Search: |
;176/76,78,81 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1041355 |
|
Feb 1964 |
|
GB |
|
1025703 |
|
Apr 1966 |
|
GB |
|
Primary Examiner: Nelson; Peter A.
Attorney, Agent or Firm: Edwards; Robert J. Kelly; Robert
H.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. In a nuclear reactor fuel assembly having a plurality of
generally equal diameter fuel elements and at least one larger
diameter guide tube longitudinally extending in parallel and
laterally spaced in a uniform square pitch array having equal
center to center distance by intersecting grid members, said
lateral spacing defining longitudinal coolant flow channels having
a first spaced distance between adjacent fuel elements and a second
smaller spaced distance between an adjacent fuel element and a
guide tube, the improvement which comprises an integral
intermittent projection extending the outside surface of the guide
tube into an adjacent coolant flow channel so as to divert some of
the flow from the adjacent coolant flow channel into a gap defined
by said second smaller spaced distance between the guide tube and
an adjacent fuel element.
2. An improved guide tube as recited in claim 1 wherein a plurality
of the integral intermittent projections are disposed at spaced
intervals about the perimeter of the guide tube.
3. An improved guide tube as recited in claim 1 or 2 wherein a
plurality of the integral projections are longitudinal spaced at
intervals along the length of the guide tube.
4. An improved guide tube as recited in claim 3 wherein the
integral projections are disposed at an angle of inclination with
respect to the longitudinal axis of the guide tube.
5. An improved guide tube as recited in claim 4 wherein the angle
of inclination with the longitudinal axis is between fifteen and
thirty degrees.
6. An improved guide tube as recited in claim 4 wherein the guide
tube is cylindrical and four integral projections are equidistantly
spaced about the circumference of the guide tube at longitudinally
spaced intervals.
7. An improved guide tube as recited in claim 5 wherein the guide
tube is cylindrical and four integral projections are equidistantly
spaced about the circumference of the guide tube at longitudinally
spaced intervals.
Description
TECHNICAL FIELD
The invention relates to fuel assemblies for nuclear reactors and,
more particularly, to an improved guide tube for fuel assemblies
used in water-cooled nuclear reactors.
BACKGROUND ART
In water-cooled nuclear reactors, the reactor core in which the
fission chain is sustained generally contains a multiplicity of
fuel element assemblies, also known as fuel assemblies, that are
identical in mechanical construction and mechanically
interchangeable in any core fuel assembly location. Each fuel
assembly is designed to maintain its structural integrity during
reactor heatup, cool-down, shut-down and power operations including
the most adverse set of operating conditions expected throughout
its lifetime. Design considerations for reactor operation include
the combined effects of flow induced vibration, temperature
gradients, and seismic disturbances under both steady state and
transient conditions.
Each fuel assembly typically contains a plurality of thin elongated
fuel elements, a number of spacer grid assemblies, guide tubes, an
instrumentation tube, and end fittings.
The fuel elements used in current applications are known as fuel
rods and comprise cylindrical UO.sub.2 fuel pellets stacked end to
end within a thin walled tube (cladding), often having spring
loaded plenums at each end of the tube, that is hermetically sealed
with end caps or plugs. The fuel rod cladding is made from a
material, such as a zirconium alloy, which has good neutron
economy, i.e. a low capture cross section. Depending upon the
position of a fuel assembly within the core, a number of elongated
cylindrical guide tubes, arranged in parallel with fuel rods, are
used variously to provide continuous sheath guidance for axially
translatable control rods, axial power shaping rods, burnable
poison rods, or orifice rods. Sufficient internal clearance is
provided to permit coolant flow through the guide tubes to limit
the operating temperatures of the absorber materials which may be
inserted therein, and to permit rod insertion and withdrawal
motions therewithin during all phases of reactor operation. The
guide tubes have a larger cross-section or diameter than the fuel
rods.
Each fuel rod transfers nuclear fission generated heat to
circulating coolant water, circulating through parallel flow
passages or coolant channels between the adjacent parallelpiped
fuel rods, guide tubes and the instrumentation tube. The coolant
channels are associated with an effective flow area transverse to
the channel length. The various types of coolant channels are
alternatively defined by the flow area between adjacent fuel rods
(known as unit channels), by the flow area between a guide tube or
instrument tube and adjacent fuel rods (known as guide tube
channels), or by the flow area between fuel rods and an adjacent
flow barrier such as the thermal shield of the reactor. The flow
area of a guide tube channel is smaller than the flow area of a
unit channel due to the larger cross-section of the guide
tubes.
In each fuel assembly, fuel rods, guide tubes and instrumentation
tube are typically supported in a square array at intervals along
their lengths by spacer assemblies that maintain the lateral
spacing between these components in an open-lattice arrangement.
Each spacer assembly is generally composed of a multiplicity of
slotted rectangular grid plates arranged to intersect and interlock
in an egg-crate fashion to form cells through which the fuel rods
and guide tubes extend in a parallel array. Illustratively, the
grid plates may be of the type, such as described in U.S. Pat. No.
3,665,586 by F. S. Jabsen and assigned to The Babcock & Wilcox
Company, which have indentations that laterally extend essentially
perpendicular to the longitudinal axes of the fuel rods into those
cells which have fuel rods for engagement and support of the fuel
rods. These spacer grids typically accommodate and support the
larger control rod guide tubes and the instrument tube in cells not
having such indentations. Despite the difference in the diameter of
the fuel rods relative to that of the guide tubes or
instrumentation tube, all of these parallelpiped components are
arrayed in a uniform square pitch, that is have equal center to
center distance, within the fuel assembly.
The spacer assemblies maintain a necessarily precise spacing
between fuel rods in order to avoid neutron flux peaks and regions
of abnormally high temperature (hot spots) where burnout, i.e.
severe local damage to the fuel rods, could occur. The spacer
assemblies assure the mechanical stability that is essential to
preclude the distortions which may be otherwise caused by flow
induced vibrations, pressure differences, and thermal stress.
Coolant typically flows upwardly through the flow channels parallel
to the surrounding members. Since the flow areas of the channels
differ, the flow rate is not the same in each type of flow
channel.
The design of the reactor core is limited by the heat transfer rate
from the fuel to the coolant. The limiting or "critical" heat flux
is defined by the onset of the departure from nucleate boiling
(DNB). This condition marks the transition into an area of low heat
transfer coefficient and a very high fuel element surface
temperature which can eventually lead to burnout. DNB can occur if
the fuel element heat flux is too great for a given coolant flow.
Reactor core design criteria, therefore, are partly based on the
establishment of a maximum permissible heat flux in the so-called
"hot channel" which is a fraction of the calculated burnout flux.
Safety margins between the maximum permissible heat flux and the
critical heat flux, characterized as "minimum DNB ratios," are set
for the various flow channel types in order to provide an adequate
margin of safety under all conditions.
Experimental studies indicate that critical heat flux values in
fuel element assemblies are generally lower in the smaller guide
tube channels than in unit channels. Thus, the guide tube channels
are, in the sense, a limiting factor in reactor operation,
particularly within the "gap" defining the closest spacing between
a guide tube and adjacent fuel rod.
SUMMARY OF THE INVENTION
The invention is directed to an improved guide tube which increases
the critical heat flux in a guide tube channel within a fuel
assembly of the type described.
In a preferred embodiment, the guide tube surface is integrally
projected into the adjacent flow channels at spaced intervals about
the perimeter of the guide tube and longitudinally along its
length. The integral projections, which are angularly disposed with
respect to the axis of the guide tube, intercept and direct some of
the coolant from the mainstream of the adjacent flow channels into
the gaps between the guide tube and adjacent fuel elements, thereby
increasing the flow in the gaps.
The various features of novelty which characterize the invention
are pointed out with particularity in the claims annexed to and
forming a part of this specification. For a better understanding of
the invention, its operating advantages and specific objects
attained by its use, reference should be had to the accompanying
drawings and descriptive matter in which there is illustrated and
described a preferred embodiment of the invention .
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, forming a part of this specification,
and in which reference numerals shown in the drawings designate
like or corresponding parts throughout the same,
FIG. 1 is an elevation view, partly broken away and partly in
section, of a fuel assembly;
FIG. 2 is a plan view of a portion of a spacer grid assembly;
FIG. 3 is a partial elevation of a guide tube made in accordance
with a preferred embodiment of the invention;
FIG. 4 is a vertical section of part of a fuel assembly
incorporating guide tubes made in accordance with the
invention;
FIG. 5 is a plan view of part of an array of fuel elements and a
guide tube spaced as by a spacer assembly in accordance with the
invention;
FIG. 6 is a schematic representation of a flow area (as illustrated
by the shaded area) of a coolant channel; and
FIG. 7 is a schematic representation of a flow area (as illustrated
by the shaded area) of another coolant channel.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a fuel element assembly 10, oriented with its
longitudinal axis in the vertical plane, including a lower end
fitting 11, and upper end fitting 12, a plurality of elongated
cylindrical fuel elements 13, hollow cylindrical guide tubes 14, a
central instrumentation tube (not shown) and spacer grid assemblies
15. The fuel elements 13 and guide tubes 14 are laterally spaced
and supported, on a uniform square pitch, in a generally square
array by the spacer grid assemblies 15. The extremities of the
guide tubes 14 are mechanically fixed to the lower 11 and upper 12
end fittings in a known manner, e.g. as shown in U.S. Pat. No.
3,828,868.
As shown in FIG. 2, each spacer grid assembly 15 is composed of
grid plates 21, 22 which are slotted and fitted together in an
"egg-crate" fashion to form fuel element cells 23 and guide tube
cells 24 (only one of which is shown) through which the fuel
elements and guide tubes will extend in a parallel array. The
spacer grid plates 21, 22 are generally of the type described in
U.S. Pat. No. 3,665,586 and, as is shown in FIG. 2, have
indentations 25 laterally extending into the cells 23 which will
contain fuel elements 13 for engagement and support of the fuel
elements. The lengthwise edges of the grid plates 21, 22 are
provided with arcuate saddles 26 in the wall portions of the plates
that form the cells 24 through which the guide tubes 14 (not shown
in FIG. 2) extend. The guide tubes 14 have larger cross-sections
than the fuel elements 13, and the arcuate saddles 26 are shaped to
conform to the shape of the guide tubes which are cylindrical in
the preferred embodiment.
A guide tube 14 constructed in accordance with a preferred
embodiment of the invention is illustrated in FIG. 3. A plurality
of integral intermittent ridges 30 radially extend the guide tube
surface at circumferentially and longitudinally spaced intervals.
The ridges are preferably oriented at an angle 26 of inclination to
the fuel assembly axis ranging from fifteen to thirty degrees. In
the preferred embodiment, four circumferentially spaced ridges 30
are formed at longitudinally aligned and spaced intervals along the
guide tube length.
An exemplary longitudinal orientation of the ridges within a fuel
assembly is shown in FIG. 4. It is preferred that the ridges be
spaced and angled so as to swirl, such as is illustrated by flow
arrows 33, the most water into the gap 40 to achieve the lowest
critical heat flux for the smallest angle of inclination with
respect to the longitudinal axis. In this regard, it is not desired
to divert flow from a guide tube channel into nearby unit channels
for mixing purposes or the like as this represents a loss in the
quantity of flow which can be diverted into the gap. A small angle
of inclination and length of ridge 30 is desirable in order to
minimize resistance to flow and to provide added assembly
flexibility, that is, permit insertion of a guide tube into an
initial assembly containing fuel elements and spacer grids
assembled by techniques such as are disclosed in U.S. Pat. No.
3,933,583 wherein elongated elements traversing opening in the grid
plates are rotated to flex the cell walls having indentations and
permit insertion of a fuel rod without damage. By minimizing the
length and angle of the ridges, the guide tube can be oriented with
each ridge disposed in a corner of a cell 24 so that the guide
tubes can be inserted through the cell for assembly. Thus, the
guide tubes may be inserted into a spacer assembly having cells for
receiving the guide tubes, each of which is arrayed amongst
surrounding adjacent cells for the receipt of smaller diameter fuel
elements, all the cells in the spacer assembly being arranged in an
equal square pitch, after the surrounding cells have been loaded
with fuel elements.
In FIG. 5, a guide tube 14, constructed in accordance with the
invention is illustrated in a plan sectional view with adjacent
fuel elements 13 within a grid structure of the type shown in FIG.
2. As is shown schematically in FIG. 6, unit channels 31 are
defined by the effective coolant flow area between adjacent fuel
elements 13. Guide tube flow channels 32 are defined by the
effective flow area between a guide tube 14 and adjacent fuel
elements 13 as is shown in FIG. 7. Numeral 41 refers to the gap
between fuel elements.
The reactor coolant generally flows within the unit channel 31 and
guide tube flow channels 32 axially parallel to the fuel elements
13 and guide tubes 14. Each ridge 30 diverts some coolant flow from
the guide tube flow channels into the gap 40 between the guide tube
14 and an adjacent fuel element 13 thereby increasing the coolant
flow rate within the gap 40. In the preferred embodiment, as
illustrated in FIG. 4, at least one longitudinal level of ridges 30
are disposed directly upstream of each spacer grid assembly. The
increase in flow rate within the gap 40, correspondingly increases
the allowable heat flux for a given minimum DNB ratio. The ridges
also increase the local turbulence levels.
The hydraulic effects of guide tube ridges in accordance with the
invention is further illustrated by, but not limited to, the
following example:
EXAMPLE
The increase in gap velocity has been demonstrated by hydraulic
testing of simulated fuel element assembly arrangements. The
simulated fuel elements and guide tubes were arrayed in a square
pitch having a center to center distance of 0.503-inches. Fuel rod
outside diameter was 0.379-inches and the guide tube outside
diameter was 0.465-inches. Hence, the gap between a fuel rod and
guide tube was 0.081-inches. The spacer assembly utilized contained
a plurality of plates having a 0.015-inch thickness arranged in
intersecting and interlocking fashion to form an "egg-crate" type
lattice of cells. The following two configuration tested (each
ridge was 0.020-inches wide and 0.037-inches high):
______________________________________ 1 2
______________________________________ Rows of Ridges Between
Spacer Assemblies 4 8 Distance Between Rows(inches) 4 2 Inclination
of Ridge to longitudinal axis(degrees) 15 22 Length of
Ridge(inches) 0.596 0.301 Gap/Bundle, average velocity ratio* 0.87
0.89 ______________________________________ *The gap to overall
tube bundle average velocity ratio in a simulated assembly without
ridges was 0.84.
The length of the ridges in the foregoing example limits the
feasibility of insertion of guide tubes into a partially assembly
unit. It is believed, however, that such guide tube, if constructed
with ridges having a 0.187-inch length, a 0.136-inch width, and a
0.037-inch height, oriented at a twenty-two degree angle from the
longitudinal axis and spaced every two inches along the length of
the guide tube, would be more than adequate for achieving the
purposes described herein.
The hydraulic-thermal benefit of the guide tube ridges represents a
favorable balance between two competing hydraulic effects. The area
of the smaller guide tube channels inherently leads to higher
pressure drops or lower flow rates relative to the larger parallel
unit channels. The lower flow rate affects both the temperature
rise of the coolant and of the fuel element surface thereby leading
to critical heat flux limitations. The introduction of guide tube
ridges into the guide tube channels promotes flow diversion from
these channels. A net benefit occurs only if the local modification
of the flow pattern within the guide tube channels results in an
increased flow through the narrow gaps between the guide tube and
adjacent fuel element in which DNB is believed to initiate. As the
frontal area and angle of the ridges is increased to make them more
effective devices for directing flow through the gaps, the flow
blockage in the critical channels increase flow diversion. Thus,
the hydraulic-thermal benefit of the ridges rests on a delicate
balance. In this regard, it should be understood that it is not
desired to utilize the guide tube ridges for generally mixing flow
within the fuel element assembly as such would not efficiently
increase the flow rate in the gap.
The guide tube ridges described herein may be formed, for example,
by dimpling the wall of a hollow guide tube into an external die
cavity by internal hydraulic pressure.
* * * * *